To study and compare, on the ultrastructural level, the morphology and the formation and extrusion of secretory material of mucous acinar cells of the sublingual gland with seromucous acinar cells of the submandibular gland of the golden hamster, Mesocricetus auratus, and to compare these cells with the acinar cells of analogous glands in the guinea pig, Cavia porcellus, through the use of general and cytochemical E.M. methods in order to (1) elucidate the secretory mechanisms of mucous salivary gland cells, (2) more clearly characterize seromucous cells, and (3) find the best salivary gland model and method for future studies of abnormal or pathological development, differentiation, and secretion of mucous salivary gland tissues.